1. Field of the Invention
The present Invention relates to a film bulk acoustic resonator having a cavity, a filter circuit and a method for manufacturing a film bulk acoustic resonator.
2. Description of the Related Art
Wireless technology has achieved remarkable development, and further development targeting high-speed wireless transmission is now in progress. At the same time, higher frequencies are more readily attainable, along with increases in the amount of transmittable information. With respect to more highly functional mobile wireless devices, there are strong demands for smaller and lighter components, and components, such as filters previously embedded as discrete components, are being integrated.
In light of these developments, one of the components drawing attention in recent years is a filter utilizing a film bulk acoustic resonator (FBAR). The FBAR is a resonator using a resonance phenomenon of a piezoelectric material, similar to a surface acoustic wave (SAW) element. The FBAR is more suitable for a high frequency operation above 2 GHz, whereas a SAW element has problems handling the relevant frequency range. Since the FBAR uses the resonance of longitudinal waves in the thickness direction of a piezoelectric film, it is possible to drastically reduce the size of the element, especially the thickness thereof. In addition, it is relatively easy to fabricate the FBAR on a semiconductor substrate, such as silicon (Si). Accordingly, the FBAR can be easily integrated into a semiconductor chip.
The FBAR is provided with cavities above and below a capacitor, in which a piezoelectric film is sandwiched between a top electrode and a bottom electrode. A typical FBAR has a stacked structured of a bottom electrode, a piezoelectric film, and a top electrode on a cavity provided above a Si substrate (refer to Japanese Unexamined Patent Publication No. 2000-69594).
In a general method for manufacturing a FBAR, for example, a groove formed in the substrate is filled with a sacrificial material, and a capacitor of the FBAR is formed on the sacrificial material. The sacrificial material is selectively removed by selective etching to form a bottom cavity. Thereafter, the capacitor is sealed using a resin film and the like so as to provide a top cavity between the resin film and the capacitor. Generally, aluminum nitride (AlN) or zinc nitride (ZnO) is used as the piezoelectric film.
In a sealing process of the top cavity, the thermal load or a weight load is applied by adhesion of the resin film. Collapse of the bottom cavity immediately below the capacitor may occur. As a result, the yield rate for manufacturing a FBAR is decreased.
A resonant frequency is primarily determined by an acoustic velocity of the piezoelectric film and thicknesses of the piezoelectric film and the electrodes, such as the top and bottom electrodes. The acoustic velocity is a characteristic value peculiar to the material of the piezoelectric film. Therefore, the resonant frequency of the FBAR is adjusted by changing the thicknesses of the piezoelectric film and the electrodes. In other words, the resonant frequency of the FBAR is extremely affected by the thicknesses of the piezoelectric film and the electrodes.
For example, in a resonant frequency range of around 2 GHz, a variation of about 1% in the thickness of the piezoelectric film or the electrodes affects a variation of about 1% in the resonant frequency. In an application to a filter circuit, in order to attain desired filtration properties, it is necessary to suppress the variation of the resonant frequency to be about 1% or less. Therefore, there is a demand to suppress variations in thickness during deposition of each of the piezoelectric film and the electrodes.
However, variations in thickness during deposition is limited by performance of a deposition apparatus. Thus, it is difficult to decrease the variations in thickness during deposition. In actuality, it is difficult to suppress the resonant frequency within a permissible range of variation. Therefore, the resonant frequency is adjusted by trimming the electrodes, in which the thickness of the top and bottom electrodes is adjusted, after formation of the capacitor.
Although there are numerous different trimming methods, in all cases, the thicknesses of the electrodes are adjusted before sealing of the top cavity above the capacitor. However, the resonant frequency of the FBAR may vary because of processes after sealing of the top cavity. Thus, the yield rate for manufacturing a FBAR is decreased.